The floating frame of reference formulation (FFRF) is widely used to model flexible multibody systems, such as wind turbines. With the development of longer and more flexible wind turbine blades, it is essential that geometrical nonlinear effects are considered to accurately capture the large deflections these structures experience. A method is presented that uses the FFRF to this end by partitioning a structure into sub-bodies, each with its own floating frame. The elastic deformation of the sub-bodies is represented by Hurty/Craig-Bampton component modes, calculated from an existing solid finite element (FE) model; the use of a high-fidelity FE model allows for capturing complex structural phenomena (e.g. three-dimensional effects) that can arise in wind turbine blades. Interface modes are used to reduce the number of interface degrees of freedom, which require a special treatment of the constraint equations needed to connect the sub-bodies. Conventional rigid multipoint constraints are also analysed. It is demonstrated that this FFRF-based multibody approach can compute accurate nonlinear responses when compared to standard solid finite element models, with a good compromise between computational cost and accuracy.

Wind turbine aeroelastic simulation tools usually rely on the blade element momentum (BEM) theory to calculate aerodynamic loads and a beam finite element model for the structural response. A method to transfer distributed aerodynamic loads computed by these aeroelastic codes to a multibody reduced-order model based on the floating frame of reference formulation (FFRF) is presented. The model is based on solid finite elements, thus constituting a higher-fidelity alternative to beam elements, and the number of degrees of freedom (DOFs) is reduced using the Hurty/Craig-Bampton method, with interface reduction based on interface modes. The proposed method consists of calculating equivalent concentrated loads and applying them to the model using interpolation multipoint constraints (RBE3). Two approaches are introduced to avoid applying loads to the internal DOFs of the reduced-order model by including internal interfaces at the load application cross-sections, either described by interface modes or using a minimum strain energy formulation. Results show that adding load interfaces can improve the static response to torsional moments, but the overall increase in accuracy is not substantial; additionally, it is found that the reduced-order models built with minimum strain energy load interfaces demonstrate an increased stiffness. The methodology is also applied to a 12.6 m wind turbine blade, showcasing the better torsional response of the model when compared with standard beam models.

Beam models are often used to represent wind turbine blades in aeroelastic simulation tools. With the development of longer and more flexible blades, larger deflections are experienced, making it essential to re-assess the accuracy of such models on a regular basis. In this study, the static response of beam-like structures containing key geometric characteristics of wind turbine blades (taper, twist and initial curvature) is computed by different numerical models, to compare the accuracy of the beam model in the aeroelastic tool HAWC2 against high-fidelity three-dimensional finite element models. It is concluded that the beam model can calculate accurate results for anisotropic structures of linearly varying properties undergoing large deflections. Relative differences up to 10% in maximum tip displacement and twist angle are observed and can be explained by inaccuracies in the calculation of cross-sectional properties and cross-sectional deformation (e.g., the Brazier effect).

With the increased interest in floating offshore wind turbines, there is an increased focus on evaluating the lifetime of the auxiliary systems, such as the mooring lines and the dynamic power cables, because these systems are in-mature compared to bottom-mounted offshore wind farms. This paper proposes a conceptual reference dynamic power cable suitable for the 15 MW NREL reference turbine mounted on the UMaine floater installed at 82 m water depth. This provides a basis for discussing the life evaluation of dynamic cables obtained from aeroelastic simulation of the floating offshore wind turbine platform exposed to the environmental conditions of the South Brittany site in France. An electromagnetic-thermal finite element simulation is used to determine the current capacity of the cable and aeroelastic simulations are used for a simple assessment of the main failure modes as maximum cable tension, minimum bending radius and fatigue damage accumulation.

Abstract. A power cable that connects a floating power plant to the grid is exposed to a dynamic environment that can pose challenging design conditions for the cable. This dynamic environment is caused by the movements of the floating substructure due to a combination of wind, wave and current forces. The power cable model needs to be able to account for its bending and axial stiffness, and hydrodynamic forcing. In this publication a catenary mooring/cable line element with bending stiffness is verified with a reference tool to assure the bending stiffness of a power cable is captured correctly. By using an existing parametric design model, a simple power cable design (in terms of overall cable length) is proposed for the IEA15MW turbine with UMaine floating substructure for a site with a water depth of 82 meter. A simple geometric method is proposed to initialize this complex lazy wave cable design in a dynamic and time domain simulation environment. The hydro-servo-aero-elastic wind turbine simulation environment HAWC2 is then used to establish an estimate of how much movements the combined system (turbine, floater, mooring lines, and power cable) experiences. This study discusses the necessary steps that are to be considered when including the dynamic power cable in the analysis and when adjusting a floating reference wind turbine design to a specific site with different water depth.

Optimization of inter-array dynamic cables for Floating Offshore Wind Farms (FOWFs) using three integer linear programs and a heuristic is presented. Design optimization of fixed-bottom offshore wind is a challenging research problem but the presence of dynamic components in FOWFs adds new complexity — as the Floating Offshore Wind Turbine (FOWT), the support structure including the station-keeping system, and the floating power cables all experience dynamic movement in reaction to wind, wave and even current forcing. In this study, dynamic modeling for the response of this system is first carried out to assess the risk of potential mechanical interference between movable elements. Subsequently, safety zones constraints are defined in the optimization to ensure minimally safe conditions for operation of the combined FOWT/support-structure/cables system. Likewise, additional constraints including maximum thermal limits, tree topology without branching, and others are incorporated. The programs follow an incremental approach. Model 1 proposes a simple way to avoid mechanical interference, Model 2 adds variables modeling mooring lines anchoring, and Model 3 increases the degrees of freedom through addition of the positioning of the touchdown point where the dynamic and static sections meet at the seabed. The applicability is illustrated through realistic case studies for a reference FOWF in Europe. Results show that: (i) Modern branch-and-cut solvers are able to solve Model 2 getting the global optimum in seconds, and (ii) further cost refining can be obtained after wrapping Model 3 in the heuristic, using Model 2 as the initial design, decreasing the cost of this layout by around 1.5% in few hours through a nonrectilinear topology.

We study the dynamic loading of a wind turbine in the presence of a single wake in the wind speed range of 8 to 12 m/s and a wind direction range of 40 degrees around the wake alignment direction. Measurement data from an offshore wind farm in the North Sea is compared to simulations. Damage equivalent loads and order spectra are compared. Five wake models are used: Frandsen added turbulence model, three parametrizations of DWM, and LES wake simulations. A new parametrization of DWM with an added wake distortion model is introduced improving the wake effect on fatigue loading of tower top torsion and tilt bending moment. All DWM models compare more favourably to measurements than the added turbulence model, but only the wake distortion model allows for a conservative fatigue assessment on all channels. LES has the potential to be even more accurate in capturing the spectral response in wake, but results are preliminary as limited LES data was available.

Mesoscale modeling can be used to analyze key parameters for wind turbine load assessment in a large variety of tropical cyclones. However, the modeled wind structure of tropical cyclones is known to be sensitive to the boundary layer scheme. We analyze modeled wind speed, shear, and wind veer across a wind turbine rotor plane in the eyewall and outer cyclone. We further assess the sensitivity of wind speed, shear, and veer to the boundary layer parametrization. Three model realizations of Typhoon Megi are analyzed over the open ocean using three frequently used boundary layer schemes in the Weather Research and Forecasting (WRF) model. All three typhoon simulations reasonably reproduce the cyclone track and structure. The boundary layer parametrization causes up to 15 % differences in median wind speed at hub height between the simulations. The simulated wind speed variability also depends on the boundary layer scheme. The modeled median wind shear is smaller than or equal to 0.11 used in the current IEC (International Electrotechnical Commission) standard regardless of the boundary layer scheme for the eyewall and outer cyclone region. However, up to 43.6 % of the simulated wind profiles in the eyewall region exceed 0.11. While the surface inflow angle is sensitive to the boundary layer scheme, wind veer in the lowest 400 m of the atmospheric boundary layer is less affected by the boundary layer scheme. Simulated median wind veer reaches values up to 1.7 x 10 - 2 degrees m - 1 ( 1.2 x 10 - 2 degrees m - 1 ) in the eyewall region (outer cyclone region) and is relatively small compared to moderate-wind-speed regimes. On average, simulated wind speed shear and wind veer are highest in the eyewall region. Yet strong spatial organization of wind shear and veer along the rainbands may increase wind turbine loads due to rapid changes in the wind profile at the turbine location.

Observations of large coherent fluctuations are used to define a probabilistic model of coherent gusts with direction change. The gust model provides the joint description of the gust rise time, amplitude, and directional changes with a 50-year return period. The observed events are from a decade of measurements from a coastal site in western Denmark, making the derived gust model site specific. In conjunction with the gust model, a yaw controller is presented in this study to investigate the load implications of the joint gust variables. These loads are compared with the design load case of the extreme coherent gust with direction change (ECD) from the IEC 61400-1 Ed.4 wind turbine safety standard. Within the framework of our site-specific gust model we find the return period of the ECD to be approximately 460 years. From the simulations we find that for gusts with a relatively long rise time the blade root flapwise bending moment, for example, can be reduced by including the considered yaw controller. From the extreme load comparison of the ECD and the modeled gusts we see that by including the variability in the gust parameters the load values from the modeled gusts are between 20 % and 74 % higher than the IEC gusts.

Tropical cyclones are associated with extreme wind speeds, enhanced turbulence, vertical wind shear, and veer. All these elements increase loads acting on structures such as wind turbines, bridges, and high-rise buildings. While most studies focus on maximal wind speeds in tropical cyclones, we analyze wind shear and veer in the lowest 300 m of the atmosphere, which is relevant for wind energy applications. We use the Weather Research and Forecasting model to model and analyze the distribution and spatial structure of wind shear and veer in Typhoon Megi (2016) at different radii. We found maximal mean shear and veer in the eyewall region. Shear and veer are on average smaller in the rainbands, but their respective distribution is positively skewed due to spatially organized outliers. These outliers are associated with convective cells and downdrafts, that propagate over structures with speeds of around 30 ms⁻¹. Consequently, structures experience rapid changes in shear and veer. We further analyze vertical cross-sections through convective cells and their propagation velocity. The study highlights differences in characteristics of the low-level wind field between the eyewall region and rainbands, which suggest distinct forces acting on structures.

Most small–medium-sized turbine studies have focused on presenting new design methods and corresponding performance improvements rather than detailed dynamic investigations. This paper presents comprehensive dynamic investigations of a straight and a swept-back blade for a 100 kW turbine by performing modal analysis, dynamic load analysis, and flutter analysis. The considered load cases include steady wind and operational conditions under normal and extreme turbulence. Modal results show that although both blades have similar natural frequencies, their mode shapes are quite different due to the couplings in flapwise-torsion directions introduced by the back-swept geometry. This coupling alters the aeroelastic response of the blade, which results in different loads in the operational conditions. The load analysis results show that the blade damage equivalent fatigue loads for the swept blade are much lower (up to 29% for the flapwise bending moment and 31% for the edgewise bending moment) than the straight blade. For the ultimate loads, blade root edgewise load for the swept blade is almost 50% lower than the straight blade while the flapwise ultimate load is similar for both blades. Moreover, both blades have no aeroelastic instability near the operational conditions, and the flutter limit for the swept-back blade is lower than the straight blade.

This paper introduces a novel, transfer-learning-based approach to include physics into data-driven normal behavior monitoring models which are used for detecting turbine anomalies. For this purpose, a normal behavior model is pretrained on a large simulation database and is recalibrated on the available SCADA data via transfer learning. For two methods, a feed-forward artificial neural network (ANN) and an autoencoder, it is investigated under which conditions it can be helpful to include simulations into SCADA-based monitoring systems. The results show that when only one month of SCADA data is available, both the prediction accuracy as well as the prediction robustness of an ANN are significantly improved by adding physics constraints from a pretrained model. As the autoencoder reconstructs the power from itself, it is already able to accurately model the normal behavior power. Therefore, including simulations into the model does not improve its prediction performance and robustness significantly. The validation of the physics-informed ANN on one month of raw SCADA data shows that it is able to successfully detect a recorded blade angle anomaly with an improved precision due to fewer false positives compared to its purely SCADA data-based counterpart.

Within this work, an existing model of a Suzlon S111 2.1 MW turbine is used to estimate potential cost savings when the conventional upwind rotor concept is changed into a downwind rotor concept. A design framework is used to get realistic design updates for the upwind configuration, as well as two design updates for the downwind configuration, including a pure material cost out of the rotor blades and a new planform design. A full design load basis according to the standard has been used to evaluate the impact of the redesigns on the loads. A detailed cost model with load scaling is used to estimate the impact of the design changes on the turbine costs and the cost of energy. It is shown that generally lower blade mass of up to 5 % less than the upwind redesign can be achieved with the downwind configurations. Compared to an upwind baseline, the upwind redesign shows an estimated cost of energy reduction of 2.3 %, and the downwind designs achieve a maximum reduction of 1.3 %.

The qualitative changes in damping of the first edgewise modes when an upwind wind turbine is converted into the respective downwind configuration are investigated. A model of a Suzlon S111 2.1 MW turbine is used to show that the interaction of tower torsion and the rotor modes is the main reason for the change in edgewise damping. For the forward whirl mode, a maximum decrease in edgewise damping of 39 % is observed and for the backward whirl mode, a maximum increase of 18 % in edgewise damping is observed when the upwind configuration is changed into the downwind configuration. The shaft length is shown to be influencing the interaction between tower torsion and rotor modes as out-of-plane displacements can be increased or decreased with increasing shaft length due to the phase difference between rotor and tower motion. Modifying the tower torsional stiffness is seen to give the opportunity in the downwind configuration to account for both a favorable placement of the edgewise frequency relative to the second yaw frequency and a favorable phasing in the mode shapes.

Aero-servo-elastic analyses are required to determine the wind turbine loading for a wide range of load cases as specified in certification standards. The floating reference frame (FRF) formulation can be used to model the structural response of long and flexible wind turbine blades. Increasing the number of bodies in the FRF formulation of the blade increases both the fidelity of the structural model and the size of the problem. However, the turbine load analysis is a coupled aero-servo-elastic analysis, and computation cost not only depends on the size of the structural model, but also depends on the aerodynamic solver and the number of iterations between the solvers. This study presents an investigation of the performance of the different fidelity levels as measured by the computational cost and the turbine response (e.g., blade loads, tip clearance, tower-top accelerations). The analysis is based on aeroelastic simulations for normal operation in turbulent inflow load cases as defined in a design standard. Two 10 MW reference turbines are used. The results show that the turbine response quickly approaches the results of the highest-fidelity model as the number of bodies increases. The increase in computational costs to account for more bodies can almost entirely be compensated for by changing the type of the matrix solver from dense to sparse.

In order to ensure structural reliability, wind turbine design is typically based on the assumption of gradual degradation of material properties (fatigue loading). Nevertheless, the relation between the wake-induced load exposure of turbines and the reliability of their major components has not been sufficiently well defined and demonstrated. This study suggests a methodology that makes it possible to correlate loads with reliability of turbines in wind farms in a computationally efficient way by combining physical modeling with machine learning. It can be used for estimating the current health state of a turbine and enables a more precise prediction of the "load budget", i.e., the effect of load-induced degradation and faults on the operating costs of wind farms. The suggested approach is demonstrated on an offshore wind farm for comparing performance, loads and lifetime estimations against recorded main bearing failures from maintenance reports. The validation of the estimated power against the 10 min supervisory control and data acquisition (SCADA) power signals shows that the surrogate model is able to capture the power performance relatively well with a 1.5% average error in the prediction of the annual energy production (AEP). It is found that turbines positioned at the border of the wind farm with a higher expected AEP are estimated to experience earlier main bearing failures. However, a clear connection between the load estimations and failure observations could not be confirmed in this study. Finally, the analysis stresses that more failure data are required in future work to enable statistically significant associations of the observed main bearing lifetimes with load exposures across the wind farm and to validate and generalize the suggested approach and its associated findings.

This report describes two wind turbine models developed within the second work package (WP2) of IEA Wind Task 37 on Wind Energy Systems Engineering: Integrated RD&D. The wind turbine models aim at acting as references for future research projects on wind energy, representing a modern land-based wind turbine and a next generation offshore wind turbine. The land-based design is a class IIIA geared configuration with a rated electrical power of 3.4-megawatts, a rotor diameter of 130 meters (m), and a hub height of 110 m. The offshore design is a class IA configuration with a rated electrical power of 10.0 megawatts, a rotor diameter of 198 m, and a hub height of 119 m. The offshore turbine adopts a direct-drive generator.

Passive Bend-Twist Coupling (BTC) can be used in blades to alleviate loads and generate more Annual Energy Production (AEP). However, BTC is inherently aero-elastic, thus difficult to incorporate into the design with sequential design process. Multi-disciplinary Design Optimization (MDO) is an attractive approach for overcoming these challenges. This paper presents the re-design of a 100kW BTC rotor using the MDO rotor design package HAWTOpt2. In the preliminary design phase, MDO was used to assess the differences between elastic BTC (i.e. off-axis fibers) and geometric BTC (i.e. sweep). This work found that aero-elastic design optimization without BTC was able to achieve a 16% improvement, then with sweep a 18% improvement and with material coupling a 17% improvement. Due to the reduced stiffness of off-axis fibers, material coupled designs had more difficulty satisfying the tip deflection constraint. The geometric BTC concept was chosen for the final design. The design optimization was repeated with additional manufacturing constraints. The final design achieved a 12% improvement.